The field of the present invention is needleless hypodermic injection devices.
Various needleless hypodermic injection devices have been known and used in the past. These known devices typically use spring or compressed gas driven plungers to accelerate an injectant to a velocity sufficient to pierce through the skin and enter the underlying tissues. While these needleless injection devices may have met with varying levels of success, certain disadvantages remain. For example, the driving pressure of such devices is temperature dependent, such that the quality of an injection can depend on the ambient temperature. Such temperature dependence typically restricts use of such devices to approximately room temperature. Furthermore, these devices either require injector adjustments to deliver various dosages or they lack variable dose capability altogether.
Since saturated carbon dioxide (CO.sub.2) in a newly inserted CO.sub.2 cartridge is mostly liquid at room temperatures, if the device is placed in an inverted position, liquid CO.sub.2 can drip or run into the internal chambers and components. Then, when the device is triggered during an injection, the stray liquid flashes to gas, inadvertently providing a faster or stronger injection than desired.
Certain needleless injection devices can also inadvertently be actuated without an ampule and/or plunger in place. If sufficiently repeated this "dry" firing can damage internal parts. In addition, these devices can be inadvertently triggered if they are not provided with safety interlocks. The rush of gas during firing also, in some devices, causes a loud, short "pop" which can startle some patients, especially children. This sound can also be psychologically linked to higher injection pain levels.
Accordingly, it is an object of the invention to provide an improved needleless injection device.